I've read up more clearly on the answers here in this thread. Research, as you say, is the key to understanding how you can better write sex scenes between the sexes. For guys/males, try looking into the start of puberty, and work your way up through the teen years to about age 17. These are EXTREMELY important years of a boy's life, which will determine many things; namely his sexuality, his future sexual performance, his sexual activity as he matures, etc etc.
If I could speak on a personal note: for me, I started puberty very young; about 8, 9, or 10 years old. And I distinctly recall experiencing many things far more early than most boys probably did at my age; my first erections, first pubes, my first erotic thoughts and fantasies (VERY active for me at that age), discovering that girls were different from me in a physical sense, etc etc. These sorts of things lasted even into my Junior-High-School years, when I REALLY started paying attention to girls and trying to cop peaks down the various shirts of girls who bent over in front of me (hey, I was 14-ish),
And I think that, because I started puberty so young, that I was very...precocious and curious, and did many things that other boys (and especially girls) would only dream of (this was ages 9 to 10 for me; perhaps even younger by a year or so). These things included trying to engage the other neighborhood kids in sexual situations that I myself didn't understand at all, and urinating with friends openly in random places and laughing at the "thrill" of it (which I understand now was a sort of sexual arousal at the time).
As an adult, I look back on these times and sigh with mild embarrassment, but it's important to note such things if anyone wants to learn from these times and experiences to get into the head of boys at a young age. Again, these times (9-17 years of age) will shape a boys life forever. And understanding these times is crucially important to understanding the psychology of the Male Species.
Going back to what others in this thread have mentioned: it really all depends on the individual writing the story in how they experienced such things. Their experiences will most likely shape how they write sex scenes between guys and girls, girls and girls, or guys and guys. And understanding the broad spectrum of experiences of boys and girls is a must in order to understand the psychology of sexuality between the two, so that you can best write the appropriate scenes.
In one of my Harry Potter fan fiction stories, entitled "Cum Once, Cum Twice", I noted in the beginning of the story ("Author's Notes") that I tried to use my own sexual experiences growing up to shape how I wrote that story (it's a Harry/Ron slash; 1 chapter done so far). The fact of the matter is, that as I got deeper and deeper into the story, I found it taking the shape, not of how my own sexual experiences occurred at that age, but rather: how I wished they had occurred. In the story, I tried to show a deep affectionate emotion between Harry and Ron, that could easily be translated into a "Love" rather than just an "Affection". But again, this was based on my wishes, not necessarily my experiences. Rather, the physical aspects of the story (how Harry masturbates Ron, for example) were based on my actual experiences growing up. Not the emotional part; just the physical part. You can read the story here: http://hp.adult-fanfiction.org/story.php?no=600097862
I hope this answers some questions, or gives you some incite, into a little bit about the Male Psychology when it comes to Sexuality. If you would like to continue this with me in depth, you can email me (I'm very open about my sexuality/experiences): vladpryde@hotmail.com, or PM me here on this forum (if you can). Email works best though.
Keep in mind: I am just one guy. A broader expanse into the Male Psych should be sought if at all possible, from any viable source necessary.